There is no way to either prove or disprove that in-person voter fraud is rare.

Maybe that explains why Pennsylvania was asked to provide evidence in support of their law to the courts, and Pennsylvania said, "Um, we can't. Just trust us." Imagine a judge hearing that. "You have no evidence that it's a problem, but you're enacting a law that significantly burdens thousands of voters for the purpose of stopping that problem?"

"Yes, your honor. You see, this could help Mitt Romney win presi -- I mean, the integrity of the electoral process demands it."